1. Field of the Invention
This invention relates to manufacture and forming of plastic, and more particularly to a design and process for welding objects to a surface of the plastic.
2. Description of the Related Art
A multitude of rigid articles are formed from plastic. Many of these articles also include objects that are mounted on the plastic. One such article is a well-known plastic fuel tank. Plastic fuel tanks provide a reservoir of fuel for engines and other fuel consuming devices in land, water and air vehicles. Plastic fuel tanks and other similar articles may be produced with blow molding or thermoforming techniques using high-density polyethylene (HDPE) plastic in monolayer or coextruded form. The plastic used for plastic fuel tanks also includes a hydrocarbon barrier to prevent passage of fuel and associated vapors there through.
Objects in the form of fuel system components are usually added to the plastic fuel tank to create a fuel system. The components may include valves, hoses, pumps, level sensors, structural supports, etc. Typically, some of these components are installed inside the plastic fuel tank by cutting service holes in the tank. In addition, some of the components are installed outside the tank requiring additional holes, grooves and/or recesses.
Recent changes in government regulations have reduced the amount of allowable fuel vapor emissions from fuel tanks. One way to reduce allowable fuel vapor emissions from plastic fuel tanks is through minimization of breaches in the hydrocarbon barrier. Reduction in the number of holes in the plastic fuel tank through internalization of fuel system components minimizes breaches. In addition, internalization of components retains any vapor emissions from the components themselves within the hydrocarbon barrier. One way to internalize fuel system components is to insert the components during manufacture of the plastic fuel tank.
Fuel system components inserted during manufacturing may be fixedly mounted to the interior walls of the plastic fuel tank while the walls are in a molten state. The mounting method is referred to as a hot plate weld process. The process involves embedding a rigid bracket included on the component in the interior wall. Warming the bracket with a hot plate may occur prior to contacting the interior wall to ensure adhesion between the bracket and the wall. The bracket is a thick, flat continuous plate or a flat plate with a waffle pattern thereon. The plate is partially, or completely, submerged in the molten plastic forming the wall. The component is maintained in position until the plastic fuel tank cools and the wall becomes rigid.
Because of temperature differences between the interior wall and the bracket, stresses may develop at the interface between the bracket and the wall as the plastic fuel tank cools. In addition, following manufacture when the plastic fuel tank retains fuel, non-uniform swelling resulting from absorption of fuel by the bracket and the interior wall may create similar stresses. The stresses can lead to failure of the bond formed between the bracket and the wall. Failure of the bond may result in fractures to the wall and/or decoupling of the fuel system component from the wall.
In addition, during manufacturing when the bracket is embedded in the interior wall, the material forming the wall is displaced. Displacement of the material may result in a breach of the hydrocarbon barrier and/or a thin spot in the wall. Further, the displaced material forms a notch feature on the interior wall resembling a ridge or a bead in the area adjacent to the bracket. The notch feature creates a discontinuity in the otherwise continuous plane formed by the wall. The discontinuity introduces forces perpendicular to any tensile/compression forces in the wall. As such, failure of the mounting and/or the interior wall may occur at the notch feature when the wall is placed under a tensile or compression load.
Accordingly, a process and system is needed for mounting objects on plastic that reduces stresses in the bond between the object and the plastic and does not compromise the structural integrity of the plastic.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the embodiments described below include a system and process utilizing an adapter to mount an object on plastic while the plastic is in a molten state. The adapter is capable of absorbing stresses by external forces as well as those stresses that may arise as the plastic and the adapter independently shrink and swell. In addition, the adapter is couple to the plastic with a surface weld. The surface weld avoids excessive displacement of the plastic, while minimizing creation of notch features or other structures detrimental to structural integrity.
The adapter includes a body, a coupling mechanism and at least two feet. The body couples the coupling mechanism with the feet. The coupling mechanism couples an object to the adapter. The feet are formed to extend away form the body and include a weld zone for contacting the surface of the plastic. Upon being placed on the surface of the plastic, the weld zone rises in temperature and reaches a molten state. Upon reaching the molten state, the weld zone and the surface of the plastic melt together to bond the object to the plastic.
In one embodiment, the adapter is used to couple at least one fuel system component to a wall of a plastic fuel tank to minimize holes and other breaches of a hydrocarbon barrier formed by the wall. In this embodiment, the adapter, with the fuel system component coupled thereto, is advanced into contact with the surface of the wall while the wall is in a molten state. A weld interface is formed when the weld zone on each of the feet and the wall melt together. The adaptor not only minimizes stresses developed while the wall and the feet cool, but also minimizes stresses created when swelling occurs in the presence of fuel.
An interesting feature of the adapter is the formation and operation of the feet. The feet are formed of flexible material to absorb stresses induced by independent shrinkage and swelling, as well as dynamic loading, of the plastic and/or the adaptor. The feet of one embodiment are formed of plastic resin that becomes progressively thinner toward the weld zone. The relatively thin portion of the feet within the weld zone quickly reaches the molten state due to low thermal mass. Conversely, the relatively thick portion of the feet do not quickly reach a molten state and therefore provide more robust structural support during installation and following cool down.
Another interesting feature of the adapter is the weld zone. The weld zone on each of the feet includes a relatively flat surface and a lip. The relatively flat surface includes substantial surface area to provide a significant contact with the surface of the plastic. The significant contact provided by the substantial surface area maintains the adaptor on the surface and provides a strong bond once a weld occurs. In addition, the substantial surface area limits the penetration depth of the adapter into the surface of the plastic and provides stability for objects coupled to the adapter. The lip avoids compromising the surface of the plastic by presenting a non-invasive edge. The non-invasive edge minimizes both the formation of notch features and the displacement of the molten plastic forming the surface.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.